Method and apparatus for making a fibrous electret web using a wetting liquid and an aqueous polar liquid

ABSTRACT

A method for imparting an electrostatic charge to a nonwoven fibrous web  20.  The fibrous web  20  is contacted with a liquid solution that includes water and a non-aqueous water soluble component, followed by drying  31.  The resulting dried product is an electret article  43  that could be used as an air filter in, for example, a respirator  50.

[0001] This is a continuation of U.S. application Ser. No. 09/415,291filed Oct. 8, 1999.

[0002] The present invention relates to a method of charging a fibrousweb by wetting it with a wetting agent and then contacting it with anaqueous polar liquid, followed by drying. The invention also pertains toan apparatus that is useful for carrying out the inventive method.

BACKGROUND

[0003] Electrically-charged nonwoven webs are commonly used as filtersin respirators to protect the wearer from inhaling airbornecontaminants. U.S. Pat. Nos. 4,536,440, 4,807,619, 5,307,796, and5,804,295 disclose examples of respirators that use these filters. Theelectric charge enhances the ability of the nonwoven web to captureparticles that are suspended in a fluid. The nonwoven web captures theparticles as the fluid passes through the web. The nonwoven webtypically contains fibers that comprise dielectric—that is,nonconductive—polymers. Electrically-charged dielectric articles areoften referred to as “electrets”, and a variety of techniques have beendeveloped over the years for producing these products.

[0004] Early work relating to electrically-charging polymer foils isdescribed by P. W. Chudleigh in Mechanism of Charge Transfer to aPolymer Surface by a Conducting Liquid Contact, 21 APPL. PHYS. LETT.,547-48 (Dec. 1, 1972), and in Charging of Polymer Foils Using LiquidContacts, 47 J. APPL. PHYS., 4475-83 (October 1976). Chudleigh's methodinvolves charging a polyfluoroethylene polymer foil by applying avoltage to the foil. The voltage is applied through a conducting liquidthat contacts the foil surface.

[0005] An early-known technique for making a polymeric electret infibrous form is disclosed in U.S. Pat. No. 4,215,682 to Kubic and Davis.In this method, the fibers are bombarded with electrically-chargedparticles as they issue from a die orifice. The fibers are created usinga “melt-blowing” process, where a stream of gas, which is blown at highvelocity next to the die orifice, draws out the extruded polymericmaterial and cools it into a solidified fiber. The bombarded melt-blownfibers accumulate randomly on a collector to create the fibrous electretweb. The patent mentions that filtering efficiency can be improved by afactor of two or more when the melt-blown fibers areelectrically-charged in this fashion.

[0006] Fibrous electret webs also have been produced by charging themwith a corona.

[0007] U.S. Pat. No. 4,588,537 to Klaase et al., for example, shows afibrous web that is continuously fed into a corona discharge devicewhile positioned adjacent to one major surface of a substantially-closeddielectric foil. The corona is produced from a high-voltage source thatis connected to oppositely-charged thin tungsten wires. Anotherhigh-voltage technique for imparting an electrostatic charge to anonwoven web is described in U.S. Pat. No. 4,592,815 to Nakao. In thischarging process, the web is brought into tight contact with asmooth-surfaced ground electrode.

[0008] Fibrous electret webs also may be produced from polymer films orfoils, as described in U.S. Pat. Re. Nos. 30,782, Re. 31,285, and Re.32,171 to van Turnhout. The polymer films or foils are electrostaticallycharged before being fibrillated into fibers that are subsequentlycollected and processed into a nonwoven fibrous filter.

[0009] Mechanical approaches too have been used to impart an electriccharge to fibers.

[0010] U.S. Pat. No. 4,798,850 to Brown describes a filter material thatcontains a mixture of two different crimped synthetic polymer fibersthat have been carded into a fleece and then needled to form a felt. Thepatent describes mixing the fibers well so that they becomeelectrically-charged during the carding. The process disclosed in Brownis commonly referred to as “tribocharging”.

[0011] Tribocharging also can occur when high-velocity uncharged jets ofgases or liquids are passed over the surface of a dielectric film. InU.S. Pat. No. 5,280,406, Coufal et al. disclose that when jets of anuncharged fluid strike the surface of the dielectric film, the surfacebecomes charged.

[0012] A more recent development uses water to impart electric charge toa nonwoven fibrous web (see U.S. Pat. No. 5,496,507 to Angadjivand etal.). Pressurized jets of water or a stream of water droplets areimpinged onto a nonwoven web that contains nonconductive microfibers tocreate the electric charge. The resulting charge providesfiltration-enhancing properties. Subjecting the web to an air coronadischarge treatment before the hydrocharging operation can furtherenhance charging.

[0013] Adding certain additives to the web has improved the performanceof electrets. An oily-mist resistant electret filter media, for example,has been provided by including a fluorochemical additive in melt-blownpolypropylene microfibers; see U.S. Pat. Nos. 5,411,576 and 5,472,481 toJones et al. The fluorochemical additive has a melting point of at least25° C. and a molecular weight of about 500 to 2500.

[0014] U.S. Pat. No. 5,908,598 to Rousseau et al. describes a methodwhere an additive is blended with a thermoplastic resin to form afibrous web. Jets of water or a stream of water droplets are impingedonto the web at a pressure sufficient to provide the web withfiltration-enhancing electret charge. The web is subsequently dried. Theadditives may be (i) a thermally stable organic compound or oligomer,which compound or oligomer contains at least one perfluorinated moiety,(ii) a thermally stable organic triazine compound or oligomer whichcontains at least one nitrogen atom in addition to those in the triazinegroup, or (iii) a combination of (i) and (ii).

[0015] Other electrets that contain additives are described in U.S. Pat.No. 5,057,710 to Nishiura. The polypropylene electrets disclosed inNishiura contain at least one stabilizer selected from hindered amines,nitrogen-containing hindered phenols, and metal-containing hinderedphenols. The patent discloses that an electret that contains theseadditives can offer high heat-stability. The electret treatment wascarried out by placing the nonwoven fabric sheet between a needle-likeelectrode and an earth electrode. U.S. Pat. Nos. 4,652,282 and 4,789,504to Ohmori et al. describe incorporating a fatty acid metal salt in aninsulating polymer to maintain high dust-removing performance over along period of time. Japanese Patent Kokoku JP60-947 describes electretsthat comprise poly 4-methyl-1-pentene and at least one compound selectedfrom (a) a compound containing a phenol hydroxy group, (b) a higheraliphatic carboxylic acid and its metal salts, (c) a thiocarboxylatecompound, (d) a phosphorous compound, and (e) an ester compound. Thepatent indicates that the electrets have long-term storage stability.

[0016] A recently-published U.S. patent discloses that filter webs canbe produced without deliberately post-charging or electrizing the fibersor the fiber webs (see U.S. Pat. No. 5,780,153 to Chou et al.). Thefibers are made from a copolymer that comprises: a copolymer ofethylene, 5 to 25 weight percent of (meth)acrylic acid, and optionally,though less preferably, up to 40 weight percent of an alkyl(meth)acrylate whose alkyl groups have from 1 to 8 carbon atoms. Five to70% of the acid groups are neutralized with a metal ion, particularly anion of zinc, sodium, lithium, or magnesium, or a mixture of these. Thecopolymer has a melt index of 5 to 1000 grams (g) per 10 minutes. Theremainder may be a polyolefin such as polypropylene or polyethylene. Thefibers may be produced through a melt-blowing process and may be cooledquickly with water to prevent excess bonding. The patent discloses thatthe fibers have high static retention of any existing or deliberate,specifically induced, static charge.

SUMMARY OF THE INVENTION

[0017] The present invention provides a new method for making a fibrouselectret web. In brief summary, the method comprises: wetting a fibrousweb, which web comprises nonconductive fibers, with a wetting agent;saturating the wetted web in an aqueous polar liquid; and substantiallydrying the web. The fibrous web may be a woven web or a nonwoven web,and it may be used as a filter element in a finished article such as arespirator or filter cartridge.

[0018] The present method differs from known charging methods in thatthe web is wetted with a wetting agent before being saturated with anaqueous polar liquid. The inventors discovered that the wetting step isbeneficial in that it can allow a better performing filter to beprovided as measured by the Quality Factor parameter described below.The wetting step may increase the measured charge density of the fibrousweb and thus enable the better performance to be obtained.

[0019] As used in this document:

[0020] “aqueous” means that the aqueous polar liquid contains at leastabout 10% water by volume.

[0021] “electric charge” means that there is charge separation.

[0022] “fibrous” means possessing fibers and possibly other ingredients.

[0023] “fibrous electret web” means a web that contains fibers and thatexhibits a quasi-permanent electric charge.

[0024] “liquid” means the state of matter between a solid and a gas.

[0025] “nonconductive” means possessing a volume resistivity of about10¹⁴ ohm•cm or greater at room temperature (22° C.).

[0026] “nonwoven” means a structure or portion of a structure in whichfibers are held together by a means other than weaving.

[0027] “polar liquid” means a liquid that has a dipole moment of atleast about 0.5 Debye and that has a dielectric constant of at leastabout 10.

[0028] “polymer” means an organic material that contains repeatinglinked molecular units or groups, regularly or irregularly arranged.

[0029] “polymeric” means containing a polymer and optionally otheringredients.

[0030] “polymeric fiber-forming material” means a composition thatcontains a polymer, or that contains monomers capable of producing apolymer, and possibly contains other ingredients, and that is capable ofbeing formed into solid fibers.

[0031] “quasi-permanent” means that the electric charge resides in theweb under standard atmospheric conditions (22° C., 101,300 Pascalsatmospheric pressure, and 50% humidity) for a time period long enough tobe significantly measurable.

[0032] “saturating” means wetting the web with the maximum, orsubstantially the maximum, amount possible of a liquid.

[0033] “web” means a structure that is significantly larger in twodimensions than in a third and that is air permeable.

[0034] “wetting” means contacting or coating substantially all thesurface area of the web that is desired to be wetted.

[0035] “wetting liquid” means a liquid that meets the Wetting Testdescribed below and that dissolves in the aqueous liquid that is used tosaturate the web.

BRIEF DESCRIPTION OF THE DRAWING

[0036]FIG. 1 is a partially-broken schematic side view of an apparatus10 for wetting and drying a fibrous web 20 in accordance with thepresent invention.

[0037]FIG. 2 is a partially-broken schematic side view of an alternateapparatus 10′ for wetting a web 20 using pressure-driven flow inaccordance with the present invention.

[0038]FIG. 3 is a partially-broken schematic side view of anotheralternate apparatus 10″ for wetting a web 20 using pressure inaccordance with the present invention.

[0039]FIG. 4 is an example of a filtering face mask 50 that can utilizean electret filter medium that has been produced in accordance with thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0040] In the present invention, an electrostatic charge is imparted toa fibrous web by wetting it with a wetting agent, saturating it with anaqueous polar liquid, and drying it substantially. The web may bepartially dried after wetting with the wetting agent. In one embodiment,the aqueous polar liquid is water. Intimate contact between the webfibers and the aqueous polar liquid may help maximize the electriccharge that is imparted to the fibers.

[0041] Nonwoven fibrous electret webs that have been produced inaccordance with the present invention exhibit at least a quasi-permanentelectric charge. Preferably, the nonwoven fibrous electret webs exhibita “persistent” electric charge, which means that the electric chargeresides in the fibers, and hence the nonwoven web, for at least thecommonly-accepted useful life of the product in which the electret isemployed.

[0042] One test for determining filtration performance of a fibrous webis known as the DOP Penetration and Pressure Drop Test, discussed below.The test involves forcing dioctyl phthalate (DOP) particles through thefibrous web and measuring the penetration of the particles through theweb and the pressure drop across the web. From the measured DOPpenetration and pressure drop, a quality factor (QF) may be calculated.The filtration efficiency of an electret can be generally estimated froman Initial Quality Factor, QF_(i). An Initial Quality Factor, QF_(i), isa Quality Factor QF that has been measured before the nonwoven fibrouselectret web has been loaded—that is, before the web has been exposed toan aerosol that is intended to be filtered.

[0043] Preferred nonwoven fibrous electret webs that are producedaccording to the invention may possess sufficient electric charge toenable the product to exhibit a QF_(i) value of greater than 0.2(millimeters (mm) H₂O)⁻¹, more preferably greater than 0.4 (mm H₂O)⁻¹,still more preferably greater than 0.7 (mm H₂O)⁻¹, and even morepreferably greater than 0.9 (mm H₂O)⁻¹ when tested according to the DOPPenetration and Pressure Drop Test described below. The initial qualityfactor of a nonwoven fibrous electret web of the invention preferablyexceeds, by at least a factor of 2, the QF_(i) value of an untreated webof essentially the same construction, and more preferably by a factor ofat least 10.

[0044]FIG. 1 schematically illustrates a method for wetting andsaturating a fibrous web 20. As shown, the fibrous web 20 is directed toa first mechanism 21 that is adapted to wet the fibrous web 20. The web20 moves through a series of rollers to a first vessel 22 that containswetting liquid 24. A nip that comprises rollers 25, 26 compresses andreleases the fibrous web 20 while it is submerged in the liquid 24. Whenthe fibrous web 20 re-expands, the wetting agent 24 can better enter theinterstitial spaces between the fibers to fully wet the web 20. The nipis beneficial to the wetting step because it assists in removing gasfrom the web.

[0045] After emerging from the first vessel 22, the web 20 then isdirected to a second mechanism 27 that is adapted to saturate thefibrous web 20. Web 20 enters a second vessel 28 that contains anaqueous polar liquid 30, which saturates the web 20 in vessel 28, and inso doing makes intimate contact with the fibers in the web 20.

[0046] Once the web has been saturated with the aqueous polar liquid, itcan be removed from the second vessel 28 so that it can be dried usingdrying system 31. To dry web 20, it can be directed through a ringer 32that includes mating rollers 34 and 36. Rollers 34 and 36 squeeze excessliquid from the web 20 before the web passes to an active dryingapparatus that includes moisture-removing elements 40, 42 disposed onopposing sides of the web 20.

[0047] The active drying apparatus may be an external source thatconsumes supplied energy for purposes of encouraging all moisture toleave the web. An active drying apparatus may include a heat source suchas a flow-through oven, a vacuum source, or an air source such as aconvective air apparatus, i.e., a stream of a drying gas. These dryingmechanisms may or may not be used in conjunction with mechanicalmechanisms such as a centrifuge or rollers to squeeze the polar liquidfrom the fibrous web. Alternatively, a passive drying mechanism, such asambient air drying, may be used to dry the fibrous web—although airdrying is generally not practical for high speed manufacturingrequirements. The invention contemplates essentially any operation orapparatus that is capable of encouraging moisture to leave the webwithout causing significant structural damage to the final product. Theresulting electret web can then be cut into sheets, rolled for storage,or formed into various articles, such as respirators or filters.

[0048] The web can be transported through the apparatus by essentiallyany device that is capable of moving the web from the mechanism 21 tothe second mechanism 25 first and then to the drier 31. A driven rolleris an example of a transport that may be suitable for this purpose, aswell as a conveyor, belt, or nip.

[0049] Upon being dried, the nonwoven web possesses sufficient electriccharge to qualify as an electret 43. The resulting electret web 43 mayalso be subjected to further charging techniques that might furtherenhance the electret charge on the web or might perform some otheralteration to the electret charge that could possibly improve filtrationperformance. For example, the nonwoven fibrous electret web could beexposed to a corona charging operation after (or perhaps before)producing an electret using the process described above. The web couldbe charged, for example, as described in U.S. Pat. No. 4,588,537 toKlaase et al., or as described in U.S. Pat. No. 4,592,815 to Nakao.Alternatively—or in conjunction with the noted charging techniques—theweb could also be further hydrocharged as described in U.S. Pat. No.5,496,507 to Angadjivand et al. The charge of the fibrous electret webalso may be supplemented using charging techniques disclosed in thecommonly assigned U.S. Pat. No. 6,375,886 and U.S. patent applicationSer. No. 09/416,216 entitled Method of Making a Fibrous Electret WebUsing A Nonaqueous Polar Liquid filed Oct. 8, 1999.

[0050]FIG. 2 illustrates an alternate embodiment for wetting and/orsaturating the fibrous web 20. Similar to the embodiment shown in FIG.1, the web 20 travels sequentially from a mechanism 21′ that wets theliquid, to a mechanism 25 that saturates the web, and then to amechanism 31 that dries the web. In this embodiment, however, a vacuumbar 38 encourages the wetting liquid 24 to flow through the fibrous web20 by creating a low pressure condition on one side of the web 20. Thevacuum bar 38 is hollow and is permeable to liquid flow on the sideclosest to the web 20. The interior of the vacuum bar 38 is held at apressure sufficiently lower than the vessel 22 so that the wettingliquid will flow through the web 20 and into the vacuum bar 38.Equipment that uses submerged vacuum bars is marketed by TUE-ESCALEIndus. of Flowery Branch, Ga., U.S.A.

[0051]FIG. 3 illustrates another alternate embodiment in which thefibrous web is sequentially wetted in a first stage 21″, followed bybeing saturated in a second stage 25, which in turn is followed by adrying step 31. At the first mechanism 21″, the fibrous web 20 isexposed to the wetting liquid 24 under a high static pressure. Thepressurized vessel 22′ has a cover 44 that has a pair of slots 46through which the fibrous web 20 can travel. The elevated pressure invessel 22′ can be maintained or controlled by adding the wetting liquidthrough port 48, as necessary. As the web enters the vessel 22′, any gastrapped in the fibrous web 20 is compressed and occupies a smallervolume. The wetting liquid 24 can flow into the web 20 as the gas iscompressed.

[0052] In lieu of the nip rollers 25, 26, the vacuum bar 38, or thepressurized vessel 22′ described above, sonic or ultrasonic vibrationscould be used to wet web 20 with the wetting liquid. A sonic orultrasonic generator may be used to vibrate the wetting liquid 24 whilethe web 20 is immersed in the liquid. The vibrations should be ofsufficient amplitude to cause any gas trapped in the web 20 to break upinto small bubbles that will be easily displaced from the web by thewetting liquid.

[0053] Alternatively, the fibrous web may be sprayed with the wettingagent and/or a polar aqueous liquid using the method and apparatusdisclosed in U.S. Pat. No. 5,496,507 to Angadjivand et al. Essentiallyany apparatus or method that helps remove gas from the web iscontemplated for use in achieving adequate web wetting. Although thewhole web is shown being wetted and saturated in the Figures, this isnot necessary for practicing the present invention. It may be desirable,for example, to wet and saturate only portions of a web to create a webthat has selected areas that act as an electret.

[0054] The relative ease with which a given fibrous web can be wet isdependent on the surface energy of the fibrous web and the surfacetension of the wetting liquid. Less work is required to wet a fibrousweb with a wetting liquid that has a surface tension that issubstantially less than the surface energy of the web, particularly whencompared to the work that is required to wet a fibrous web with wettingliquid that has a surface tension that is equal to or greater than thesurface energy of the web. The wetting liquid preferably has a surfacetension that is less than the surface energy of the fibrous web, andmore preferably is at least 5 dynes per centimeter (dynes/cm) less thanthe surface energy of the fibrous web.

[0055] A liquid that qualifies as a “wetting liquid” is one thatsatisfies the Wetting Test. The Wetting Test is performed as follows.First, a dry test specimen is placed on a smooth, horizontal surface. Asmall drop—approximately 5 millimeters in diameter (0.05 ml volume)—isplaced on the test specimen using a dropping bottle. The drop isobserved for 10 seconds. If the drop substantially soaks into the webwithin this time frame, then the liquid qualifies as a wetting liquid.Preferably the drop will soak into the web—that is, satisfy the WettingTest—within about 5 seconds, and more preferably within about 2 seconds.The wetting liquid also is capable of dissolving in the aqueous liquidthat is used to saturate the web. The wetting liquid should be capableof yielding a single phase when dissolved in the aqueous liquid.

[0056] The surface tension of the aqueous polar liquid plays animportant role in imparting an electric charge to the fibrous web.Effective charging may be difficult to establish unless the surfacetension of the aqueous polar liquid is greater than the surface energyof the fibrous web. The surface tension of the polar aqueous liquid ispreferably 5 dynes/cm greater than the surface energy of the fibrous weband more preferably 10 dynes/cm greater than the surface energy of thefibrous web. Polypropylene is a polymer that is commonly used to createmelt-blown fibrous webs. It has a surface energy of about 30 dynes/cm.In webs that have more than one type of fiber, the fiber with the highersurface energy might be charged more than fibers with a lower surfaceenergy.

[0057] A wetting liquid can facilitate the wetting of fibrous webs withan aqueous polar liquid by removing trapped gas. Useful wetting liquidsmay include solutions of surfactants, such as detergents, in polaraqueous liquids. The surfactant can be a nonionic surfactant such ast-octylphenoxypolyethoxyethanol, an anionic surfactant such as sodiumlauryl sulfate, or a cationic surfactant such asalkyldimethylbenzylammonium chloride. Other wetting liquids may includewater-miscible solvents that can wet a nonwoven web in pure form or aspart of an aqueous solution due to the low surface tension of thesolvent. Preferably, the wetting liquid can be an alcohol such asisopropanol, ethanol, methanol, 2-propanol, or a ketone such as acetone,or combinations of the alcohols and/or ketones. The wetting liquid mayalso include the use of alcohols or ketones by themselves or inconjunction with water as an aqueous solution.

[0058] The method of the invention can be carried out in a batchwiseprocess, which involves a stepwise soaking of the web in the wettingliquid, followed by being submerged in an aqueous polar liquid for adesignated period of time, removing the web from the aqueous polarliquid, and then allowing the web to dry. Energy or mechanical work canbe applied to the wetting liquid, aqueous polar liquid, and/or thefibrous web to improve wetting and/or saturation as discussed above. Theuse of these procedures can enable the electret web to be producedcontinuously.

[0059] For applications where the steps of wetting or saturating areperformed by mechanical methods such as spraying the wetting liquidand/or aqueous polar liquid onto the web, or agitating the web in thepresence of these fluids, the velocity of the wetting liquid and/oraqueous polar liquid relative to the nonwoven web is preferably lessthan about 50 meters/second (m/s), and more preferably less than about25 m/s. Lower velocities are generally desirable to avoid damaging theweb, which may occur when the web is relatively delicate—for example, aweb that contains melt-blown microfibers. Nonwoven fibrous webs thatcontain microfibers can be damaged if excess energy or mechanical workis used to achieve liquid wetting or saturation. Care therefore shouldbe taken when handling a microfiber-containing web.

[0060] Preferably, the wetting liquid is in contact with the nonwovenweb for at least 0.001 seconds, more preferably for at least 1 to 10seconds in a continuous process before contacting the web with theaqueous polar liquid. The aqueous polar liquid is preferably wetted onthe fibers of the fibrous web for at least 0.001 seconds, and typicallyfor 1 second to 5 minutes.

[0061] Aqueous polar liquids suitable for use in the present method havea dipole moment of at least 0.5 Debye, and more preferably at least 0.75Debye, and still more preferably at least 1.0 Debye. The dielectricconstant is at least 10, preferably at least 20, and more preferably atleast 40. Aqueous polar liquids that have higher dielectric constantstend to create webs that show greater filtration performanceenhancement. Examples of nonaqueous components that may be used in theaqueous polar liquids include methanol, ethylene glycol, dimethylsulfoxide, dimethylformamide, acetonitrile, and acetone, among others.The aqueous polar liquid and the wetting agent preferably do not leave aconductive, non-volatile residue that would mask or otherwise dissipatecharge on the web.

[0062] Water has a dipole moment of about 1.85 Debye and a dielectricconstant of about 78 to 80. The aqueous polar liquid comprises at least10 volume % water, more preferably at least 30 volume % water, stillmore preferably at least 50 volume % water, and even more preferably atleast 80 volume % water. One hundred % water may also be used. Water isa preferred polar liquid because it is inexpensive, and no significantdangerous or harmful vapors or pollutants are generated when it contactsthe molten or semi-molten fiber-forming material. Preferably purifiedwater, made through, for example, distillation, reverse osmosis, ordeionization, is used in the present invention rather than simply tapwater. Purified water is preferred because non-pure water can hindereffective fiber charging.

[0063] Webs suitable for use in this present invention may be made froma variety of techniques, including air laid processes, wet laidprocesses, and melt blown processes such as described in Van A. Wente,Superfine Thermoplastic Fibers, 48 INDUS. ENGN. CHEM. 1342-46 and inReport No. 4364 of the Naval Research Laboratories, published May 25,1954, entitled Manufacture of Super Fine Organic Fibers by Van A. Wenteet al. Microfibers, particularly meltblown microfibers, are particularlysuitable for use in fibrous webs that are used as filters. “Microfiber”means fiber(s) that have an effective diameter of about 25 micrometersor less. Effective fiber diameter can be calculated using equationnumber 12 in Davies, C. N., The Separation of Airborne Dust andParticles, INST. MECH. ENGN., LONDON PROC. 1B (1952). For filteringapplications, the microfibers preferably have an effective fiberdiameter of less than about 20 micrometers, and more preferably about 1to about 10 micrometers.

[0064] Staple fibers may also be combined with the microfibers toprovide a more lofty, less dense web. Reducing web density can reducethe pressure drop across the web. Lower pressure drops are desirable inpersonal respirators because it can make the respirator more comfortableto wear. Preferably, no more than about 90 weight percent staple fibersare present, more preferably no more than about 70 weight percent. Websthat contain staple fibers are disclosed in U.S. Pat. No. 4,118,531 toHauser.

[0065] For filtration applications, the nonwoven web preferably has abasis weight less than about 500 grams/meter² (g/m²), more preferablyabout 5 to about 400 g/m^(2,) and still more preferably about 20 to 100g/m^(2.) In making melt-blown fiber webs, the basis weight can becontrolled, for example, by changing either die throughput or collectorspeed. The thickness of the nonwoven web for many filtrationapplications is about 0.25 to about 20 millimeters (mm), more typicallyabout 0.5 to about 4 mm. The nonwoven web preferably has a solidity (aunitless parameter that defines the solids fraction in the web) of atleast 0.03, more preferably about 0.04 to about 0.15, and still morepreferably about 0.05 to about 0.1. The inventive method can impart agenerally uniform charge distribution throughout the resulting nonwovenweb, without regard to basis weight, thickness, or solidity of theresulting media.

[0066] Active particulate also may be included in the electret webs forvarious purposes, including sorbent purposes, catalytic purposes, andothers. U.S. Pat. No. 5,696,199 to Senkus et al. describes variousactive particulate that may be suitable. Active particulate that hassorptive properties—such as activated carbon or alumina—may be includedin the web to remove organic vapors during filtration operations. Theactive particulate may be present in general in amounts up to about 80volume percent of the contents of the web. Particle-loaded nonwoven websare described, for example, in U.S. Pat. No. 3,971,373 to Braun, U.S.Pat. No. 4,100,324 to Anderson, and U.S. Pat. No. 4,429,001 to Kolpin etal.

[0067] Polymers, which may be suitable for use in producing fibers thatare useful in this invention, include thermoplastic organicnonconductive polymers. These polymers are generally capable ofretaining a high quantity of trapped charge and are capable of beingprocessed into fibers, such as through a melt-blowing apparatus or aspun-bonding apparatus. The term “thermoplastic” refers to a polymericmaterial that softens when exposed to heat. The term “organic” means thebackbone of the polymer includes carbon atoms. Preferred polymersinclude polyolefins, such as polypropylene, poly-4-methyl-1-pentene,blends or copolymers containing one or more of these polymers, andcombinations of these polymers. Other polymers may include polyethylene,other polyolefins, polyvinylchlorides, polystyrenes, polycarbonates,polyethylene terephthalate, other polyesters, and combinations of thesepolymers and other nonconductive polymers.

[0068] The fibers may be made from these polymers in conjunction withother suitable additives. The fibers also may be extruded or otherwiseformed to have multiple polymer components. See U.S. Pat. No. 4,729,371to Krueger and Dyrud and U.S. Pat. Nos. 4,795,668, and 4,547,420 toKrueger and Meyer. The different polymer components may be arrangedconcentrically or longitudinally along the length of the fiber tocreate, for example, bicomponent fibers. The fibers may be arranged toform a macroscopically homogeneous web, which is a web that is made fromfibers that each have the same general composition.

[0069] The fibers used in the invention do not need to contain ionomers,particularly metal ion neutralized copolymers of ethylene and acrylic ormethacrylic acid or both to produce a fibrous product suitable forfiltration applications. Nonwoven fibrous electret webs can be suitablyproduced from the polymers described above without containing 5 to 25weight percent (meth)acrylic acid with acid groups partially neutralizedwith metal ions.

[0070] The performance of the electret web can be enhanced by includingadditives in fiber-forming material before contacting it to a polarliquid. Appropriate additives can be added to the fiber-forming materialbefore the fibers are produced. Preferably, an “oily-mist performanceenhancing additive” is used in conjunction with the fibers or thefiber-forming materials. An “oily-mist performance enhancing additive”is a component which, when added to the fiber-forming material, or forexample, is placed on the resulting fiber, is capable of enhancing theoily aerosol filtering ability of the nonwoven fibrous electret web.

[0071] Fluorochemicals can be added to the polymeric material to enhanceelectret performance. U.S. Pat. Nos. 5,411,576 and 5,472,481 to Jones etal. describe the use of a melt-processable fluorochemical additive thathas a melt temperature of at least 25° C. and has a molecular weight ofabout 500 to 2500. This fluorochemical additive may be employed toprovide better oily-mist resistance. One additive class that is known toenhance electrets that have been charged with water jets are compoundsthat have a perfluorinated moiety and a fluorine content of at least 18%by weight of the additive—see U.S. Pat. No. 5,908,598 to Rousseau et al.An additive of this type is a fluorochemical oxazolidinone described inU.S. Pat. No. 5,411,576 as “Additive A” of at least 0.1% by weight ofthe thermoplastic material.

[0072] Other possible additives are thermally stable organic triazinecompounds or oligomers, which compounds or oligomers contain at leastone nitrogen atom in addition to those in the triazine ring. Anotheradditive known to enhance electrets charged by jets of water isChimassorb™ 944 LF (poly[[6-(1,1,3,3,-tetramethylbutyl)amino]-s-triazine-2,4-diyl][[(2,2,6,6-tetramethyl-4-piperidyl) imino]hexamethylene [(2,2,6,6-tetramethyl-4-piperidyl) imino]]), availablefrom Ciba-Geigy Corp. Chimassorb™ 944 and “Additive A” may be combined.Preferably the additive Chimassorb™ and/or the above additives arepresent in an amount of about 0.1% to about 5% by weight of the polymer;more preferably, the additive(s) is present in an amount from about 0.2%to about 2% by weight of the polymer; and still more preferably ispresent in an amount from about 0.2 to about 1 weight % of the polymer.Some other hindered amines are also known to increase thefiltration-enhancing charge imparted to the web site.

[0073] Fibers that contain additives can be quenched after shaping aheated molten blend of the polymer and additive—followed by annealingand charging steps—to create an electret article. Enhanced filtrationperformance can be imparted to the article by making the electret inthis manner—see U.S. patent application Ser. No. 08/941,864, whichcorresponds to International Publication WO 99/16533. Additives also maybe placed on the web after its formation by, for example, using thesurface fluorination technique described in U.S. patent application Ser.No. 09/109,497, filed Jul. 2, 1998 by Jones et al.

[0074] The polymeric fiber-forming material has a volume resistivity of10¹⁴ ohm•cm or greater at room temperature. Preferably, the volumeresistivity is about 10¹⁶ ohm•cm or greater. Resistivity of thepolymeric fiber-forming material can be measured according tostandardized test ASTM D 257-93. The fiber-forming material used to formthe melt blown fibers also should be substantially free from componentssuch as antistatic agents, which could increase the electricalconductivity or otherwise interfere with the fiber's ability to acceptand hold electrostatic charges.

[0075] Nonwoven webs of this invention may be used in filtering masksthat are adapted to cover at least the nose and mouth of a wearer.

[0076]FIG. 4 illustrates a filtering face mask 50 that may beconstructed to contain an electrically-charged nonwoven web that isproduced according to the present invention. The generally cup-shapedbody portion 52 is adapted to fit over the mouth and nose of the wearer.A strap or harness system 54 may be provided to support the mask 50 onthe wearer's face. Although a single strap 56 is illustrated in FIG. 4,the harness 54 may employ more than one strap 56 and may come in avariety of configurations—see, for example, U.S. Pat. No. 4,827,924 toJapuntich et al., U.S. Pat. No. 5,237,986 to Seppalla et al., and U.S.Pat. No. 5,464,010 to Byram.

[0077] Examples of other filtering face masks where nonwoven fibrouselectret webs may be used include U.S. Pat. No. 4,536,440 to Berg, U.S.Pat. No. 4,807,619 to Dyrud et al., U.S. Pat. No. 4,883,547 toJapuntich, U.S. Pat. No. 5,307,796 to Kronzer et al., and U.S. Pat. No.5,374,458 to Burgio. As shown in these patents, the nonwoven fibrouselectret web is used as a filter in the cub-shaped mask body. Theelectret filter media also may be used, for example, in a filtercartridge for a respirator, such as the filter cartridge disclosed inU.S. Pat. No. Re. 35,062 to Brostrom et al. or in U.S. Pat. No.5,062,421 to Burns and Reischel. Mask 50 thus is presented forillustration purposes only, and use of the present electret filter mediais not limited to the embodiment disclosed.

[0078] Applicants believe that the present charging method deposits bothpositive and negative charge onto the fibers such that the positive andnegative charge is randomly dispersed throughout the web. Random chargedispersal produces an unpolarized web. Thus, a nonwoven fibrous electretweb produced in accordance with the present invention may besubstantially unpolarized in a plane normal to the plane of the web.Fibers that have been charged in this manner ideally exhibit the chargeconfiguration shown in FIG. 5C of U.S. patent application Ser. No.08/865,362. If the fibrous web is also subjected to a corona chargingoperation, it would exhibit a charge configuration similar to theconfiguration shown in FIG. 5B of that patent application. A web, formedfrom fibers charged solely using the present method, typically hasunpolarized trapped charge throughout the volume of the web.“Substantially unpolarized trapped charge” refers to a fibrous electretweb that exhibits less than 1 μC/m² of detectable discharge currentusing TSDC analysis, where the denominator is the electrode surfacearea. This charge configuration can be shown by subjecting the web tothermally-simulated discharge current (TSDC).

[0079] Thermally-stimulated discharge analysis involves heating anelectret web so that the frozen or trapped charge regains mobility andmoves to some lower energy configuration to generate a detectableexternal discharge current. For a discussion on thermally-stimulateddischarge current, see Lavergne et al., A review of Thermo-StimulatedCurrent, IEEE ELECTRICAL INSULATION MAGAZINE, vol. 9, no. 2, 5-21, 1993,and Chen et al., Analysis of Thermally Stimulated Process, PergamonPress, 1981.

[0080] An electric charge polarization can be induced in a web that hasbeen charged according to the present invention by elevating thetemperature to some level above the glass transition temperature (T_(g))of the polymer, which is the temperature where a polymer changes to aviscous or rubbery condition from a hard and relatively-brittle one. Theglass-transition temperature, T_(g), is below the polymer's meltingpoint (T_(m)). After raising the polymer above its T_(g), the sample iscooled in the presence of a DC electric field to freeze-in thepolarization of the trapped charge. Thermally-stimulated dischargecurrents can then be measured by reheating the electret material at aconstant heating rate and measuring the current generated in an externalcircuit. An instrument useful for performing the polarization andsubsequent thermally-stimulated discharge is a Solomat TSC/RMA model91000 with a pivot electrode, distributed by TherMold Partners, L.P.,Thermal Analysis Instruments of Stamford, Conn.

[0081] The discharge current is plotted on the y axis (ordinate) againstthe temperature on the x axis (abscissa). The peak (current maximum)position and shape of the discharge current are characteristics of themechanism by which the charges have been stored in the electret web. Forelectret webs that contain a charge, the peak maximum and shape arerelated to the configuration of the charge trapped in the electretmaterial. The amount of charge produced in the outside circuit due tomovement of the charge inside the electret web to a lower energy stateupon heating can be determined by integrating the discharge peak(s).

[0082] Fibrous electret webs of the invention exhibit a Measured ChargeDensity, which is a measure of the relative amount of unpolarizedtrapped charge. The Measured Charge Density can be ascertained using theprocedure described below. Fibrous electret webs of the inventionpreferably exhibit a Measured Charge Density of at least 0.3microcoloumbs per square meter (μC/m²), more preferably a MeasuredCharge Density of at least 0.6 μC/m², and still more preferably at least0.9 μC/m². In some instances, Measured Charge Density can exceed 7μC/m².

[0083] Advantages and other properties and details of this invention arefurther illustrated in the following Examples. Although the examplesserve this purpose, the particular ingredients and amounts used andother conditions are not to be construed in a manner that would undulylimit the scope of this invention. For example, while the Examplesillustrate inventive methods that produce products on an individualbasis, the processes can also be performed continuously. The Examplesselected for disclosure below are merely illustrative of how to make apreferred embodiment of the invention and how the articles may generallyperform.

EXAMPLES

[0084] Sample Preparation

[0085] The nonwoven web was prepared generally as described by Van A.Wente, 48 INDUS. & ENGN. CHEM. 1342-46 (1956). The thermoplastic resinwas ESCORENE 3505G polypropylene (available from Exxon Corp.) unlessotherwise specified. The extruder was a Berstorff 60 millimeter (mm), 44to 1, eight barrel zone, co-rotating twin screw extruder. When anadditive was incorporated in the resin, it was prepared as a 10-15weight % concentrate in a Werner Pfleiderer 30 mm, 36 to 1 corotatingtwin screw extruder. The water was purified by reverse osmosis anddeionization. The basis weight of the web was about 54-60 grams/meter²,unless otherwise specified.

[0086] DOP Penetration and Pressure Drop Test

[0087] The DOP Penetration and Pressure Drop Test was performed byforcing dioctyl phthalate (DOP) 0.3 micrometer mass median diameterparticles through a sample of the nonwoven web which is 11.45 centimeter(4.5 inches) diameter at a rate of 42.5 liters/minute. The face velocityon the sample was 6.9 centimeters per second (cm/s). The DOP particleswere generated using a TSI No. 212 sprayer (available from TSI of St.Paul, Minn.) with four orifices and 207 kilo pascals (kPa) (30 psi) ofclean air at a concentration of between about 70 and about 110milligrams/meter³. The samples were exposed to the aerosol of DOPparticles for 30 seconds. The penetration of the DOP particles throughthe samples was measured using an optical scattering chamber, PercentPenetration Meter Model TPA-8F available from Air Techniques Inc. ofBaltimore, Md. The pressure drop (ΔP) across the sample was measuredusing an electronic manometer and reported in millimeters of water.

[0088] The DOP penetration and pressure drop values were used tocalculate a quality factor “QF value” from the natural log (ln) of theDOP penetration by the following formula:

QF[1/mm H₂O]=−(ln((DOP Pen %)/100))/ΔP[mm H₂O]

[0089] All samples tested below were tested for an Initial QualityFactor, QF_(i).

[0090] As indicated above, higher initial QF values are indicative ofbetter filtration performance.

[0091] Measured Charge Density

[0092] Electric charge polarizations were induced in four samples ofeach web by (i) heating each sample to a temperature of 100° C., (ii)poling each sample in the presence of a DC field of 2.5 kilovolts permillimeter (KV/mm) at 100° C. for poling periods of 5, 10, 15 or 20minutes, and (iii) cooling each sample to −50° C. in the presence of theDC field to “freeze” the trapped and poled charge in the web. Each websample was then reheated so that the frozen charge regained mobility andmoved to a lower energy state, generating a detectable externaldischarge current. Specifically, after poling in the DC field mentionedabove, each web sample was reheated from about −50° C. to about 160° C.at a heating rate of about 3° C./minute. The external current generatedwas measured as a function of temperature. The uncorrected measuredcharge density of each sample was determined by calculating the areaunder the discharge peaks and dividing the result by the area of thesample. The uncorrected measured charge density of each web was setequal to highest value of uncorrected measured charge density among thefour samples analyzed for each web. Polarization and subsequentthermally-stimulated discharge was performed using a Solomat TSC/RMAModel 91000 with a pivot electrode, distributed by TherMold Partners,L.P., Thermal Analysis Instruments of Stanford, Conn. The measuredcharge density arising from trapped, unpolarized charge can bedetermined by analyzing an untreated web of the same composition andphysical characteristics. The measured charge density of the treated webis determined by subtracting the uncorrected measured charge density ofthe untreated from the uncorrected measured charge density of thetreated web.

Example 1 and Comparative Examples C1-C2

[0093] A nonwoven web that contained blown polypropylene microfibers wasprepared as described above using ESCORENE 3505G polypropylene(available from Exxon Corp.). The effective fiber diameter of thesamples was about 8-9 μm. Individual samples about 22 inches by about 11inches (55.9 cm by 27.9 cm) were cut from this web. One sample wassoaked in isopropanol, removed, hung up in a fume hood while excessisopropanol dripped off. The sample was then immersed in about 8 litersof deionized water for about 10 to 20 minutes, removed, passed through awringer to remove excess water, and air dried overnight. Comparativesample C1 was soaked in isopropanol, removed, passed through a wringer,and air dried overnight. Comparative sample C2 was unwetted.

[0094] Circular samples about 5.25 inches in diameter (13.3 cm) were cutfrom the samples and were subjected to the DOP Penetration and PressureDrop Test using the center 4.5 inches (11.4 cm) of each circle. QF_(i)was calculated for each sample as described above. The results of thetwo evaluations conducted for each web sample were averaged and aregiven in Table 1. TABLE 1 Effect of Aqueous Treatment on FiltrationPerformance Pressure Drop Penetration QF_(i) Example Treatment (mmwater) (%) (mm H₂O)⁻¹ 1 Isopropanol; 2.68 33.2 0.41 Water C1 Isopropanol2.72 82.9 0.069 C2 None 2.48 83.4 0.073

[0095] The data show that the nonwoven web of Example 1 wetted withisopropanol followed by saturation with water demonstrated asignificantly higher Initial Quality Factor than the comparativesamples.

[0096] Samples according to Examples 1, C1 and C2 were reproduced andevaluated for Quality Factor and Measured Charge Density as describedabove. TABLE 2 Measured Charge Density Uncorrected Measured ChargeQF_(i) Density Measured Charge Example Treatment (mm H₂O)⁻¹ (μC/m²)Density (μC/m²) 1 Isopropanol; 0.45 1.00 0.95 Water C1 Isopropanol 0.090.07 0.02 C2 None 0.09 0.05 0.0

[0097] The data of Table 2 show that the nonwoven web of Example 1wetted with isopropanol, followed by saturation with water, demonstrateda significantly higher Initial Quality Factor and Measured ChargeDensity over the comparative examples.

Example 2 and Comparative Examples C3-C4

[0098] A web that contained blown polypropylene microfibers was preparedaccording to Example 1 and Comparative Examples C1-C2, except 1 weight %of a fluorochemical oxazolidinone additive, described in U.S. Pat. No.5,411,576 as “Additive A,” was added to the polypropylene melt prior toforming the blown microfibers. Additive A has the following formula:

[0099] The web also was annealed at about 140° C. for about 10 minutes.Samples were cut and were wetted as described in Example 1 andComparative Examples C1-C2. All samples had an effective fiber diameterof about 8-9 μm and had a basis weight of about 57 grams/meter². Sampleswere cut and were evaluated for filtration performance as in theprevious examples. The results of duplicate evaluations were averagedand are given in Table 3. TABLE 3 Effect of Aqueous Treatment onFiltration Performance Polypropylene Plus Fluorochemical Pressure DropPenetration QF_(i) Example Treatment (mm water) (%) mm H₂O)⁻¹ 2Isopropanol; 2.16 14.1 0.91 Water C3 Isopropanol 2.11 42.2 0.41 C4 None2.10 85.9 0.072

[0100] The data of Table 3 show that the presence of the additiveresulted in a higher Initial Quality Factor for the samples of Examples2 and C3 than the Initial Quality Factor for the samples of Examples 1and C1.

[0101] Samples according to Examples 2, C3, and C4 were reproduced andwere evaluated for Initial Quality Factor and Measured Charge Density.TABLE 4 Measured Charge Density for Fibers that Contained PolypropylenePlus Fluorochemical Uncorrected Measured Measured Charge Charge QFiDensity Density Example Treatment (mm H₂O)⁻¹ (μC/m²) (μC/m²; ) 2Isopropanol; 0.72 16.6 7.65 Water C3 Isopropanol 0.17 10.58 1.63 C4 None0.14 8.95 0.0

[0102] The data in Table 4 show that the nonwoven web of Example 2wetted with isopropanol, followed by saturation with water, demonstrateda significantly higher Initial Quality Factor and Measured ChargeDensity than the web of Example 1 that did not contain the additive.

Example 3 and Comparative Examples C5-C6

[0103] A web that contained blown polypropylene microfibers was preparedas described in Example 1 and Comparative Examples C1-C2 except thatabout 0.5 wt % Chimassorb 944 LF was added to the polypropylene meltbefore forming the blown microfibers. Samples were cut and were wettedas described in Example 1 and Comparative Examples C1-C2. All sampleshad similar effective fiber diameters and basis weights. Samples werecut and were evaluated for filtration performance as in the previousexamples. The results are given in Table 5. TABLE 5 Effect of AqueousTreatment on Filtration Performance Polypropylene Plus Chimassorb ™ 944Pressure Drop Penetration QF_(i) Example Treatment (mm water) (%) (mmH₂O)⁻¹ 3 Isopropanol; 2.56 53.3 0.25 Water CS Isopropanol 2.45 83.40.074 C6 None 2.52 85.7 0.061

[0104] The data of Table 5 show that wetting the web in isopropanolfollowed by soaking in water and drying enhances the filtrationperformance of all three filter web samples relative to the unwettedsamples. Example 3 shows that water can be used to enhance filtrationperformance of nonwoven polymeric webs. Example 3 did not show animprovement over Example 1, even though it contained the Chimassorb™additive. This result is believed to have occurred because Chimassorb™is soluble in isopropanol.

[0105] All the patents and patent applications cited above, includingthose discussed in the Background, are incorporated by reference intothis document in total.

[0106] The present invention may be suitably practiced in the absence ofany limitation not explicitly described in this document.

What is claimed is:
 1. A method of making a fibrous electret web, whichmethod comprises: applying a liquid solution that contains water and anonaqueous water-soluble component to a fibrous fluid-permeable web; andthen substantially drying the web to create a fibrous fluid-permeableelectret web.
 2. The method of claim 1, and wherein the web is a fibrousnonwoven fluid-permeable electret web.
 3. The method of claim 2, whereinthe fibers are microfibers that contain polypropylene and fluorine. 4.The method of claim 1, wherein the fibrous electret web is capable ofdemonstrating a quality factor of at least 0.2 (mm H₂O)⁻¹ when testedaccording to the DOP Penetration and Pressure Drop Test.
 5. The methodof claim 1, wherein the fibrous electret web is capable of demonstratinga quality factor of at least 0.4 (mm H₂O)⁻¹ when tested according to theDOP Penetration and Pressure Drop Test.
 6. The method of claim 1,wherein the fibrous electret web is capable of demonstrating a qualityfactor of at least 0.7 (mm H₂O)⁻¹ when tested according to the DOPPenetration and Pressure Drop Test.
 7. The method of claim 1, whereinthe fibers comprise fluorine, and the fibrous electret web is capable ofdemonstrating a quality factor of at least 0.9 (mm H₂O)⁻¹ when testedaccording to the DOP Penetration and Pressure Drop Test.
 8. The methodof claim 1, wherein the fibers further comprise an oily-mist performanceenhancing component.
 9. The method of claim 1, wherein the web comprisesmicrofibers.
 10. The method of claim 9, wherein fibers contain fluorineatoms in or on the fibers.
 11. The method of claim 1, wherein the web iswetted with a wetting liquid before applying the liquid solution. 12.The method of claim 11, further comprising the step of removing excesswetting liquid before applying the liquid solution.
 13. The method ofclaim 1, wherein the liquid solution is applied to the fibrous web suchthat it saturates the web.
 14. The method of claim 1, wherein the web isair dried.
 15. The method of claim 1, wherein the web is dried byexposing the web to heat.
 16. The method of claim 1, wherein the web isdried by exposing the web to a static vacuum.
 17. The method of claim 1,wherein the web is dried by exposing the web to a stream of a heateddrying gas.
 18. The method of claim 1, wherein the web is dried bymechanically removing the liquid solution.
 19. The method of claim 1,wherein the fibers comprise polypropylene, poly-4-methyl-1-pentene, orblends or copolymers containing one or both of these materials.
 20. Themethod of claim 1, wherein the fibers comprise a polyolefin,polyvinylchloride, a polystyrene, a polycarbonate, a polyester, or ablend thereof.
 21. The method of claim 1, wherein the fibrous electretweb is substantially unpolarized in a plane normal to a plane of the webwhen subjected to thermally stimulated discharge.
 22. The method ofclaim 1, wherein the fibrous electret web exhibits substantially nodischarging current when subjected to thermally stimulated discharge.23. The method of claim 1, wherein the fibrous electret web exhibitssubstantially no net charge.
 24. The method of claim 1, wherein the webis a macroscopically homogeneous web.
 25. The method of claim 1, whereinthe fibrous electret web has an unpolarized charge.
 26. The method ofclaim 1, wherein the fibrous electret web has a Measured Charge Densityof at least 0.3 μC/m².
 27. The method of claim 11, wherein the wettingliquid satisfies the Wetting Test within 5 seconds.
 28. The method ofclaim 1, wherein the wetting liquid satisfies the Wetting Test within 2seconds.
 29. A filtration mask adapted to cover the nose and mouth of awearer comprising the fibrous electret web of claim
 1. 30. The method ofclaim 1, wherein the liquid solution has a dipole moment of at least 0.5Debye.
 31. The method of claim 1, wherein the liquid solution has adipole moment of at least 0.75 Debye.
 32. The method of claim 1, whereinthe liquid solution has a dipole moment of at least 1 Debye.
 33. Themethod of claim 1, wherein the liquid solution has a dielectric constantof at least
 10. 34. The method of claim 1, wherein the liquid solutionhas a dielectric constant of at least
 20. 35. The method of claim 1,wherein the liquid solution has a dielectric constant of at least 40.36. The method of claim 1, wherein the liquid solution has a dipolemoment of at least 0.5 Debye and has a dielectric constant of at least10.
 37. The method of claim 1, wherein the liquid solution has a dipolemoment of at least 0.75 Debye and has a dielectric constant of at least20.
 38. The method of claim 11, wherein the liquid solution and thewetting liquid do not leave a conductive, non-volatile residue on thefibrous electret web.
 39. The method of claim 1, wherein the fibers havefluorine atoms on their surfaces.
 40. The method of claim 1, wherein theresulting fibrous electret web is substantially unpolarized in a planenormal to the plane of the web.
 41. The method of claim 1, wherein thenon-aqueous water-soluble component is selected from the groupconsisting of methanol, ethylene glycol, dimethyl sulfoxide,dimethylformamide, acetonitrile, and acetone.